1. Field of the Invention
This invention relates to a semiconductor light-emitting device such as a refractive index waveguide laser or a double heterojunction light-emitting diode and a method of manufacturing the same and, more particularly, to a semiconductor light-emitting device, in which an active region is surrounded by a semiconductor layer whose forbidden band width is larger than that of the active region, wherein the active region has opposed edge surfaces serving as reflecting mirrors for oscillating a laser beam between the opposed edge surfaces and emitting the laser beam from one of the edge surfaces, and a method of manufacturing the same.
2. Description of the Related Art
Recently, various semiconductor light-emitting devices having a double heterostructure have been developed. In the semiconductor light-emitting devices of this type, it is very important to satisfy the following conditions A to C.
A. Current should be efficiently concentrated to only a light-emitting region or active region with the size thereof controlled to a very small value for improving the light-emitting efficiency.
B. Electrodes covering wide regions have to be formed to reduce the contact resistance.
C. Where high-speed modulation is required as in a light-emitting device, the area of a p-n junction should be minimized to reduce the junction capacitance.
As a well-known semiconductor light-emitting device for optical communication which more or less satisfies the above three conditions, there is a messa laser utilizing a mass transport technique (e.g., Y. Hirayama et al. "Low Temperature and Rapid Mass Transport Technique for GaInAsP/InP DFB Lasers", Inst. Phys. Conf. Ser. No. 79: Chapt. 3, Paper presented at Int. Symp. GaAs and Related Compounds, Karuizawa, Japan, 1985, p. 175,186). Such a semi- light-emitting device is called an MT laser. A method of manufacture and characteristics of the MT laser will be described below with reference to the accompanying drawings.
FIGS. 1A to 1D are sectional views schematically showing steps of manufacture of a prior art MT laser. As shown in FIG. 1A, on n-type (100) InP substrate 1 are sequentially crystal-grown approximately 3-.mu.m thick n-type InP buffer layer 2, 0.1-.mu.m thick undoped GaInAsP active layer 3 which has a composition enabling emission of light having a wavelength of 1.3 .mu.m, 1.5-.mu.m thick p-type InP clad layer 4 and 0.8-.mu.m thick p-type GaInAsP cap layer 5 which enables satisfactory ohmic contact and emits light having a wavelength of 1.15 .mu.m.
Then, as shown in FIG. 1B, the wafer is selectively etched until active layer 3 is exposed, thus forming a mesa stripe having a width of 15 .mu.m, a size permitting comparatively ready masking. The GaInAsP layer 5 may be etched by means of etchant composed of sulfric acid, hydrogen peroxide and water (4:1:1). At this time, by using hydrochloric acid for the removal of p-type InP layer 4, the etching can be automatically stopped at active layer 3 owing to the selectivity of the acid.
Subsequently, as shown in FIG. 1C, both sides of active layer 3 are etched with an etchant composed of sulfric acid, hydrogen peroxide and water (in proportions of 4:1:1) to form an active region having a width of about 1 .mu.m. At this time, InP is substantially not etched, and only GaInAsP which is a four-element mixed crystal is etched. Cap layer 5 is etched to a depth only about one-third of that of active layer 3 due to a difference in composition. In order to obtain stable fundamental transverse oscillation and low oscillation threshold current, the width of active layer 3 has to be controlled accurately to around 1 .mu.m.
Then, as shown in FIG. 1D, an InP layer is buried in the deep gap of active layer 3, having been formed by etching, to obtain a so-called buried heterostructure (BH) from considerations of light leakage of a proper amount in transversal mode and mechanical strength. In the MT laser, an MT process is used for the growth of the burried InP layer. In other words, use is made of a phenomenon that by doping phosphorus at a high temperature (670.degree. C.) and under a high pressure, InP is grown preferentially in the gap. InP can be grown more rapidly and at a lower temperature by using InCl.sub.3 as additive.
SiO.sub.2 film 6 is then formed over the entire top surface of the element as an insulating film, and a window is formed in a contact portion of this film. AuZn is then formed as p-side electrode 7 by a lift-off process and is then heated for alloying. Electrode 8 is then formed by deposition of Au-Cr on electrode 7 and insulating film 8. Further, n-side electrode 9 is formed on substrate 1, thus completing the MT laser.
With this MT laser, current can be concentrated in active region 3 by a built-in potential difference between GaInAsP of active region 3 and InP of the buried layer. In addition, since a junction is formed only in the mesa portion, the junction capacitance is low and, therefore, high-speed response can be obtained advantageously. Further, electrode 7 may be formed with its width of about 10 .mu.m.
This MT layer, however, has a problem in the controllability of the width of the active region. When the active region having a width of 15 .mu.m is selectively etched from its opposite ends for the formation of the active region with a width of about 1 .mu.m, it is difficult to stop the etching accurately to obtain a dimension of 1 .mu.m, and sometimes the entire active region is etched, thus deteriorating the yield of manufacture. The accuracy of etching is deteriorated with increase of the width of the mesa. For this reason, the mesa width can not be increased beyond 15 .mu.m. From this consideration and also from the consideration of the mask alignment margin, the mesa width of the ohmic electrode should be set to 10 .mu.m or less. Further, the mesa width imposes a lower limit on the area of the InP junction of the buried portion.
It is possible to control the area of the buried portion by controlling the time of the MT step. However, the controllability is very inferior. Therefore, the width of the buried InP junction portion can not be optionally reduced to reduce the junction capacitance while allowing light leakage in the transversal mode. This has been a great barrier for the realization of high performance. In the mean time, in order to reduce the junction capacitance and hence increase the rise voltage in the junction portion to reduce current leakage and increase the output, the carrier density of the buried junction portion should be optimized. However, in the existing MT process the carrier density is not controlled, so that the carrier density of the junction portion can not be prescribed. This poses serious problems in the design.
As described above, with the prior art MT process it is difficult to set the width of the active region accurately, which constitutes a great obstruction to the realization of a buried type semiconductor light-emitting device having high performance. Further, by reducing the area of the buried portion, the contact area is reduced to increase the contact resistance. On the other hand, by increasing the contact area, the area of the buried portion is increased to increase the junction capacitance and make it more difficult to control the width of the active region.
Moreover, the semiconductor laser device which is fabricated by the MT process has poor versatility and dictates cumbersome widing if it is to be combined for use with other auxiliary elements or passive elements such as transistors.